Less than three years after that investment, that Cambridge, MA-based company, now known as Bluebird Bio, is ready to spread its wings and take off on the Nasdaq after pricing an upsized IPO late Tuesday ahead of its range.

[Updated with early trading numbers] Bluebird sold about 5.94 million shares at $17 apiece, raising $101 million. Those numbers blow past the 5 million shares it hoped to sell at between $14 and $16 apiece when it set its range in May. Bluebird’s underwriters also have the option to buy an additional 891,716 shares at $17 apiece. Bluebird began trading on the Nasdaq under the symbol “BLUE” on Wednesday, and immediately busted out. Shares soared more than 50 percent and traded at close to $26 apiece as of 10:45 ET.

According to Bluebird’s IPO prospectus, the company has raised more than $120 million through four rounds of financing since 2010. Third Rock was Bluebird’s largest shareholder prior to the IPO, with 28.1 percent of the company’s shares, followed by TVM Capital (14.3), entities associated with Fidelity Investments (11.8 percent), Arch Venture Partners (10.6 percent), and Capital Research and Management (9.2 percent).

The IPO is emblematic not just of the appetite for biotech IPOs this year, but also of a step forward for gene therapy, a process in which healthy genes are transported, typically by a virus, into cells to treat diseases. The field has been around for two decades, but has never led to an approved product in the U.S. despite the hype.

Under the name Genetix, the company endured a 17-year-long winter before a study in 2010 showed that its approach to gene therapy was able to slow the progression of a rare disease known as childhood cerebral adrenoleukodystrophy, or CCALD, a rare genetic brain disorder, in two children. The company takes stem cells harvested from a patient’s bone marrow, inserts a healthy version of the disease-causing gene into them, grows those cells in a culture, and then puts the healthy gene into a patient in a one-time procedure.

Fast forward to 2013, and Bluebird has only increased its backing. Another big-name rare disease specialist, Shire, is now among its investors. And in March, Summit, NJ-based Celgene jumped in as well, paying Bluebird $75 million up front and potentially $225 million per product to develop cancer treatments based on its gene therapy platform. The collaboration has a three-year term, though Celgene has the option to extend it.

Even so, Bluebird still has a lot of work to do to get the clinical evidence it needs to hit the jackpot and begin selling one of its therapies. Bluebird knows the three diseases that it will target initially—CCALD; beta-thalassemia, and sickle cell disease—but it hasn’t even begun testing the current form of its most advanced therapy, for CCALD, in clinical trials yet (the 2010 study used an earlier virus as part of the process). It will begin a pivotal trial of up to 15 patients this year. The plan is to use its therapy on those patients and monitor them over two years post-treatment to see if their disease has stabilized, according to the prospectus. Bluebird has already won orphan status for the therapy from the FDA and the European Medicines Agency, and it believes good news from that trial could give it enough data to file an application for regulatory approval.

Bluebird will begin an early-stage trial of its beta-thalassemia therapy in the U.S. this year as well (one is already underway testing the therapy in patients with beta-thalassemia and sickle cell disease in France). It aims to file an investigational drug application with the FDA in 2014 to test the therapy in patients with sickle cell disease in the U.S.

The IPO cash will help to fund those clinical trials.

Bluebird is the first of two Third Rock-backed biotechs to hit the Nasdaq this year. Agios Pharmaceuticals, creating drugs that attack the mutated enzymes that feed cancer cells, filed for an IPO on June 10 with plans to raise $86 million. Third Rock was Agios’ largest shareholder before the IPO, holding 23.65 percent of its stock.

Ben Fidler is Xconomy's Deputy Biotechnology Editor. You can e-mail him at bfidler@xconomy.com Follow @benthefidler